The lines by which power is delivered to a building and by which power is distributed in a building can be and are used to transmit information between buildings and between rooms within a building. Because transformers (e.g. three-phase, double-sided single-phase, and combinations of three-phase) are used in the transmission of power, when transmitting carrier frequency current signals, particularly high frequency signals (e.g. above one hundred KHz), over the power lines, the signals will not pass efficiently through the transformers or from one phase to another or between transformers.
FIG. 1A illustrates one arrangement which has been suggested in the past for solving this problem by use of a coupling capacitor. Input power, for example at 240 volts, is delivered along a pair of power lines 10 and 12 to the primary winding 14a of a transformer 14 which develops power, for example at 120 volts, at each of two power lines 16 and 18 which are connected to the secondary 14b of the transformer, with a neutral line 20 at the center tap of the transformer secondary. Power lines 16 and 18 and neutral line 20 provide distribution of power within a building to various outlets, represented by pairs of terminals 22, 24 and 26, 28, in rooms within the building. FIG. 1B shows, by way of resistor 30, inductor 32, and capacitor 34, the characteristics of lines 16 and 20 and, by way of resistor 36, inductor 38, and capacitor 40, the characteristics of lines 18 and 20.
Information on carrier frequency current signals can be and has been transmitted between outlets (i.e. terminals 22, 24 and 26, 28) on lines 16, 20 and 18, 20 in different rooms within the building via these lines. These signals can be coupled through secondary winding 14b of transformer 14. However, because the transformer acts as a low pass filter at higher frequencies, these signals are attenuated significantly. It has been suggested that, instead of coupling the carrier frequency current signals between lines 16, 20 and lines 18, 20 through secondary winding 14b of transformer 14, these signals can be coupled between lines 16, 20 and lines 18, 20 through a capacitor 41 connected between power lines 16 and 18 to improve signal level. The use of such a capacitor for the coupling of carrier frequency current signals, however, does not preserve phase linear communications, caused in part by impedance mismatches, and, consequently, can introduce distortion and standing waves. Similar problems are likely for three-phase power input and three-phase power distribution and when coupling carrier frequency current signals between power distribution systems in different buildings.